1. Field of the Invention
This invention relates to the continuous production of nitrilotriacetonitrile.
2. Background
It is well known that an amine will react with formaldehyde and hydrogen cyanide to produce an aminonitrile. Nitrilotriacetonitrile can be produced from hexamethylenetetramine by the following reaction: ##STR1## Nitrilotriacetonitrile can be produced from ammonia or a soluble ammonium salt such as ammonium sulfonate by the following reaction: ##STR2## The rate of formation of nitrilotriacetonitrile increases with increased temperature. The reactions are highly exothermic.
The nitrilotriacetonitrile can be hydrolyzed to produce nitrilotriacetate. Nitrilotriacetate, normally as one of its water soluble salts, is used as a builder in detergent formulations.
Nitrilotriacetonitrile has been produced commercially in a batch process. However, three types of continuous processes have been disclosed in the patent literature.
U.S. Pat. No. 3,907,858 discloses a continuous process in which the reactants are brought together in a tubular plug-flow reactor under pressure, preferably autogenous pressure, of 5 to 100 psig at a temperature of at least 120.degree. C. The tubular plug-flow design was used to minimize backmixing.
Increased temperatures can cause at least two problems. First, the corrosive character of the reaction mixture increases with temperature, causing damage to the reactor and to its control systems. Second, the rate at which acidic hydrolysis of the nitrilotriacetonitrile occurs increases with temperature causing reduction of the yield. These problems become more pronounced above 120.degree. C.
U.S. Pat. No. 3,463,805 discloses a substantially adiabatic process for production of nitrilotriacetonitrile in which the reaction is permitted to exotherm producing a temperature rise of 50.degree. C. to 100.degree. C. from an initial temperature of 0.degree. C. to 130.degree. C., and an autogenous pressure rise of 1 to 3 atmospheres. The actual temperatures and pressures depend upon a number of factors, including the concentration of the reactants. A continuous process is disclosed in which this adiabatic process takes place as the reactants are pumped through an elongated reaction zone contained in an insulated reactor. The only system described uses a coiled stainless steel tubular reactor. U.S. Pat. No. 3,515,742 discloses an improvement in this adiabatic process in which heat is removed from the reacted reaction mixture by means of a heat exchanger. The heat so removed is then used to heat the reactants prior to their entry into the insulated reaction zone.
In an adiabatic process, the actual temperature and pressure rises during the reaction are determined by the temperature, concentrations, and rates of flow of the reactants entering the reactor. In order to attain a steady state temperature and pressure, these variables must be controlled very precisely. The normal fluctuations of these variables that occur during production can easily cause an upset of the delicately balanced temperature and pressure. Undesirable increases in temprature and pressure result in an increased rate of reaction, which in turn produces a further temperature increase. This self-reinforcing temperature cycle can cause the temperature and pressure to overshoot the desirable range. The disadvantages related to higher temperatures are discussed above. Additionally, broad fluctuations in temperature and pressure can pose a safety problem. The adiabatic process also can produce inconsistent results due to the lack of adequate control of the process.
U.S. Pat. No. 3,925,448 discloses production of nitrilotriacetonitrile from ammonium sulfate, formaldehyde, and hydrogen cyanide in a multistep process. The ammonium sulfate and formaldehyde are combined in a forming operation at the desired ratio, in a pump recirculation loop. The hydrogen cyanide is added in a second stage recirculating loop reactor at 93.degree. C. which is kept full and under pressure to eliminate hydrogen cyanide vapor. This was followed by a third stage storage reaction step. The yield in the Example was 90%, but this required a reaction time of 1 to 1.5 hours.
In the processes discussed above, the reactants are heated to the desired operating temperature by a combination of the hot surfaces of the reactors and heat exchangers and the heat of reaction. This results in a heating period which requires additional reactor space to accomplish. During this heating period, intermediates such as methylenebisiminodiacetonitrile are produced. Methylenebisiminodiacetonitrile can be converted to nitrilotriacetonitrile, but this conversion proceeds slowly, requiring additional reaction time to accomplish.
An object of this invention is a continuous process to produce nitrilotriacetonitrile.
A further object of this invention is a continuous process to produce nitrilotriacetonitrile in which temperature and pressure can readily be controlled, with a minimum of heat transfer.
A further object to this invention is a continuous process to produce nitrilotriacetonitrile in which the formation of byproducts and intermediates is inhibited.
Still further objects of this invention will be apparent from the discussion below.